Projection systems utilizing a liquid crystal light valve are usable in a variety of applications such as computers, video projectors and television. A projection system of this type may include a liquid crystal light valve, an illumination system for providing light to the light valve and projection optics for receiving light from the liquid crystal light valve and projecting such light toward a projection surface, such as a screen.
An important component of this system is the liquid crystal light valve which modulates the light in accordance with applied electrical signals to provide the desired image for projection onto the screen. The liquid crystal light valve may include a nematic liquid crystal cell and light polarizers. The crystals are oriented so that the light valve blocks incident light when no voltage is applied to the light polarizers, and the crystals are reoriented when a proper signal is applied so that the light valve transmits light. It is desirable for a liquid crystal light valve to have high contrast on the order of 100 to 1 where contrast is defined as the ratio of the intensity of light transmitted with the light valve turned on to the intensity of light transmitted with the light valve turned off.
A well known characteristic of liquid crystal light valves is that their ability to transmit or block light varies with the angle of incidence of the light on the valve. The angular dependence of the transmission of light through the light valve is such that there is a wide acceptance angle providing high contrast in a first direction and a narrower acceptance angle providing high contrast in a second direction which is perpendicular to the first direction. This characteristic of liquid crystal light valves is discussed, for example, in Characterizations of Viewing-Angle-Dependent Colorimetric and Photometric Performance of Color LCDS by T. G. Fiske and L. D. Silverstein, SID 93 Digest, pages 565-568 and in Tejima et al U.S. Pat. No. 4,936,657. For convenience, the first direction is hereinafter sometimes referred to as the X direction and the second direction is sometimes referred to as the Y direction. In a spatial coordinate system having X, Y and Z axes, the X and Y directions are commonly thought of being horizontal and vertical, respectively. However, because the light valve can be oriented in various different ways, unless expressly stated, the X and Y directions as used herein do not necessarily mean horizontal and vertical directions, respectively.
The X and Y directions can be defined with respect to certain components of the liquid crystal light valve. Thus, the liquid crystal light valve produces maximum contrast when the direction of the incident illumination coincides with the long axis of the liquid crystal molecules. When maximum signal voltage is applied to electrode plates of the light valve, the long axes of the liquid crystal molecules tilt nearly 90.degree. upward from the plane of the electrode. The Y direction lies in a plane which is normal to the plane of the electrodes and which contains the long axes of the liquid crystal molecules when the maximum voltage is applied. The X direction is perpendicular to the Y direction.
The prior art has recognized the importance of the small acceptance angle in the Y direction and this is shown, for example, by Goldenberg U.S. Pat. No. 4,912,614 and Duwaer U.S. Pat. No. 5,146,248. These patents seek to provide light to the liquid crystal light valve which has only a small deviation or beam spread angle with respect to collimated rays in both the X and Y directions. Simply stated, the attempt is to provide light to the liquid crystal light valve which is nearly collimated in both the X and Y directions. The provision of nearly collimated light in the Y direction provides a small deviation or beam spread angle in the Y direction and is effective to provide high contrast. However, this system does not provide as much total light to the liquid crystal light valve for a given source luminous intensity as is desired. More specifically, this system is very inefficient by not permitting additional light to be transmitted through the light valve in the X direction.
Another characteristic of a liquid crystal light valve is that the acceptance angle in the Y direction has a fixed angular bias which may be, for example, 5 to 20 degrees. This is discussed, for example, in A High-Light-Output Active-Matrix TN-LC Projector for Video and Data-Graphics Applications by P. Candry et al, SID 93 Digest pages 291-94 and Tejima et al U.S. Pat. No. 4,936,657. If the incident light falls outside the optimum acceptance angle in the Y direction as offset by the bias angle, then light leaks through the liquid crystal light valve in the off state and the contrast ratio is reduced.
To obtain high contrast, the bias angle must be taken into account and this results in the transmission of light through the liquid crystal display valve substantially at the bias angle. It is desirable to deflect the light transmitted by the liquid crystal display through a deflection angle toward the projection optics to at least partially correct for the bias angle. Tejima et al U.S. Pat. No. 4,936,657 teaches the use of a Fresnel lens to accomplish this beam deflection. A Fresnel lens has a series of discrete steps in one or both of its surfaces, and one disadvantage of this construction is that for certain applications these steps may be imaged by the projection optics.